Tunable optical sources of ultrafast pulses have found increasing use in physics, chemistry and biology. For almost two decades the most popular tunable optical source of ultrafast pulses has been the Titanium doped Sapphire or Ti:sapphire laser. This laser source possesses a combination of high average power, short pulses, and reasonably broad tunability. Typical Ti:sapphire lasers can produce about 1 W to about 4 W of average power with pulse durations on the order of 100 fs. Both picosecond pulses as well as much shorter pulses at lower power have also been demonstrated. The tuning range can extend from about 680 nm to about 1080 nm although with significantly less than about 1 W average power available at the edges of the tuning range.
During the last decade Ti:sapphire lasers and optical sources that pump Ti:sapphire lasers have been combined into a single box and engineered to produce a hands-off, fully automated tunable laser system. The advent of these user-friendly tunable ultrafast one-box laser systems has greatly broadened the number of applications accessible for these lasers. In particular, applications in biology and bio-chemistry have been well served by these sources. For example, multi-photon microscopy (MPM) uses both an ultrafast laser and a microscope to examine a specimen or sample.
In multi-photon microscopy an ultrafast source of near infrared (IR) light is used to excite the sample rather than the continuous wave (cw) source of visible light that is used in single photon systems. The samples under investigation are excited not by one photon of visible light but rather by two (or more) photons from the ultrafast near IR source. Only the portions of the sample that are at the focus of the laser beam are subject to optical radiation having sufficient intensity to experience the two photon excitation. When this portion of the sample is doped with a fluorescent dye, all the fluorescence is emitted from the small volume of the sample that is excited. For example, U.S. Pat. No. 5,034,613 issued to Denk et al., which is incorporated by reference in its entirety herein, describes a two-photon laser microscope. Advantages of this technique include improved spatial resolution, and “optical sectioning” of the sample since the excited volume extends only over a small depth in the sample. The use of the near IR wavelengths is also desirable for extended lifetime of living samples relative to visible excitation.
The popularity of tunable ultrafast sources for this application is due to the fact that many biological samples do not fluoresce on their own. As such, dyes are used to stain portions of the sample. Presently, there are dozens of dyes available depending on the sample that is to be investigated. Further, each dye has its own wavelength of optimal two-photon absorption. Thus, a tunable optical source allows great flexibility in what dyes may be used, and thus, what types of samples can be investigated.
Typically, the microscope includes a number of lenses made of glass and often at least one acousto-optic modulator for modulating the laser beam. As the ultrafast laser pulses pass through these materials, the pulse duration increases due to the dispersion. As a result, the optical signal may be temporally broadened, often by a factor of 2 or 3. This material dispersion can be pre-compensated for by placing a pair of prisms between the laser and the microscope in an appropriate configuration. Further, this dispersion compensation allows for 100 fs pulses to be incident directly on the sample. Recently, automated prism pairs have been added to the aforementioned one-box ultrafast laser systems. During use the user tunes the laser through a computer interface. In response, the laser adjusts both the wavelength and the position or orientation of the prism pair to maintain the shortest pulse at the sample.
One of the limits of MPM is that the depth of penetration of the light into the sample is restricted, often to only tens of microns. In biological samples, scattering of the fluorescence emitted by the excited portion of the sample is the limiting factor. This scattering decreases at longer wavelengths however and, thus, longer wavelength ultrafast sources can lead to greater penetration depth. There are a number of additional limits to longer wavelength excitation. For example, currently most microscopes have increasingly large losses due to the coatings on their optics at wavelengths longer than about 1300 nm. Further, typically in most biological samples there is a strong absorption due to water near 1400 nm.
Thus, it is desirable to have an ultrafast source that tunes not just from about 680 nm to about 1080 nm as the Ti:sapphire laser does, but from about 650 nm to about 1400 nm. A combination of sources has been demonstrated to cover this entire range. If the output of the Ti:sapphire laser is used to pump an optical parametric oscillator (OPO), the OPO can cover the missing wavelength range from 1080 nm to 1400 nm. Ti:sapphire pumped OPOs using KTP, RTP or CTA have been demonstrated to cover most of this wavelength range from about 1050 nm to 1330 nm. These OPOs typically produce average powers of a few hundred mW and pulse durations of 200 fs. In combination with the Ti:sapphire laser that is output from a separate port, most of the tuning range can be covered.
More recently OPOs using a periodically poled Lithium Niobate (PPLN) crystal have been demonstrated. Translating the PPLN, which typically contains a fan-shaped grating, provides tuning of the OPO. In combination with tuning the pump laser, a tuning range that covers from 1000 nm to 1600 nm with no wavelength gaps has been demonstrated. The OPO requires mirrors with high reflectivity over this entire tuning range in order to avoid having to manually replace mirrors in the middle of the tuning range. Again, in combination with the Ti:sapphire laser that is output from a separate port, all of the tuning range from 650 nm to beyond 1400 nm can be covered. Because the output of the Ti:sapphire and the OPO come from different output ports, fully automated tuning of the system with a single beam and with dispersion compensation has not yet been demonstrated.
Another alternative to cover the entire tuning range desired for MPM would be to use a shorter wavelength pump laser. Very recently an ultrafast source at 1045 nm was frequency doubled to produce ultrafast pulses in the green at 523 nm. The frequency doubled pump source was used to pump an OPO with a Type 1 BBO crystal and a tuning range of 690 nm to 980 nm was demonstrated for the signal wavelength with pulse durations of 300 fs. Idler wavelengths that covered the range from 1120 nm to >2000 nm were output from a second port. This system has a gap in the tuning range due to the degeneracy of the OPO at 1045 nm.
Thus, there is an ongoing need for a source of ultrafast pulses with an extremely broad tuning range with high average power and 100 fs pulses both for MPM and other applications. There is a further ongoing need for this tunable ultrafast source to cover the range from 680 nm to 1380 nm in and be fully automated. In addition, there is yet a further need for this automated source of ultrafast pulses to have a single output port. Lastly, there is yet another need to couple this single port ultrafast source with a fully automated dispersion compensation device.